The present invention relates to the structure of a solid state image sensor and a system using the solid state image sensor.
Conventional video input systems using solid state image sensors have been used for video cameras which record motion video data on tapes, supervisory cameras, still video cameras which record still video data on video floppy disks and digital memory media, industrial cameras, and the like. Most of these systems use area CCD image sensors.
An area CCD image sensor includes a photoelectric converter constituted by a two-dimensional array of photoelectric elements corresponding to pixels. An optical image is formed on this photoelectric converter. Signals converted into charges by the photoelectric converter are sequentially read out as pixel signals by a vertical transfer CCD and a horizontal transfer CCD.
As another solid state image sensor, a MOS type image sensor is available. The MOS type image sensor uses no CCDs for vertical and horizontal transfer. In this sensor, pixels selected by selection lines constituted by aluminum lines are read out through read lines, like a memory device. The MOS type image sensors were once actually used for video cameras. However, the MOS type image sensors were replaced with CCD image sensors because the MOS type image sensors cause larger noise than the CCD image sensors.
The MOS type image sensor, however, has characteristic features which the CCD image sensor does not have. For example, a CMOS type image sensor is driven by a single drive source unlike the CCD image sensor which is driven by multiple drive sources.
More specifically, to drive the CCD image sensor, a plurality of positive and negative power supply potentials, e.g., +20V, +15V, and −10V, are required. In contrast to this, the MOS type image sensor can be driven by a single power supply of, e.g., +5V; it requires only one power supply potential. The same power supply voltage as that used for other circuits making up an image sensing system, e.g., an amplification circuit and a control circuit, can be used for the MOS type image sensor. The number of power supplies can therefore be decreased.
The power consumption of the MOS type image sensor is also smaller than that of the CCD image sensor.
The MOS type image sensor has another characteristic feature which the CCD image sensor does not have. That is, a logic circuit, an analog circuit, an analog/digital conversion circuit, and the like can be easily formed on the sensor by using a single MOS circuit manufacturing process. It is a well-known fact that peripheral circuits, associated circuits, and the like can be easily formed on the MOS type image sensor. Prototypes of such sensors have been reported at academic meetings (e.g., ISSCC in 1996).
As described above, the MOS type image sensor has characteristic features which the CCD image sensor does not have. To make the most of these characteristic features, however, a circuit structure in the sensor which is suited to a system to be used and an interface for other circuit portions are required. If, for example, an appropriate interface is not used, a large number of pins are required to result in an increase in the chip area of the sensor or the size of the package. An increase in cost cannot therefore be avoided.
Video compression techniques for teleconferences, videophones, and the like have been standardized. With the widespread use of personal computers and communication services for personal computers, a desktop conference using personal computers will soon become a reality. Image compression techniques are also used for this purpose.
A video camera recorder or a portable video camera is used for an image capture section of such a video system. Outputs from these cameras are still analog video outputs. It is as a matter of course that in the future such cameras are connected to personal computers by digital direct coupling or incorporated therein. If a solid state image sensor incorporating a video processing circuit is available as an image sensor used for such purposes, the number of parts can be decreased. A reduction in cost can therefore be attained.
As described above, the MOS type image sensor as a solid state image sensor has many advantages over the CCD image sensor except for noise. With advances in noise suppression techniques, the MOS type image sensor has regained attention. When the MOS type image sensor is to be used as an image sensing device, a pixel selection function, an image compression function, a low speed shot control function, an image data conversion function, and the like may be implemented as MOS circuits on the same chip as that of the MOS type image sensor, in addition to an image sensing function. With this structure, only processing results can be used. As a result, a reduction in load can be attained in terms of design and manufacture of an actual system using the MOS type image sensor to take place of peripheral circuits for the above functions.
If, therefore, a solid state image sensor with video processing circuits is available as an image sensor, the number of parts can be reduced, and a reduction in cost can be attained.
If, however, these video processing circuits are simply implemented on the chip of the MOS type image sensor, a problem is posed in terms of operability when this chip is applied to a system. If, for example, a function is designed on the basis of the specifications required by the user, the resultant device becomes a single-function device. That is, a dedicated device, i.e., a device used for a special purpose, is obtained, resulting in poor versatility.
Considering the social background of an information-oriented society and the popularity of multimedia, an image capture function will be increasingly required in various fields. In addition, with increasing demand in space and energy savings, reductions in the size and power consumption of a function element are required. Under the circumstances, it is urgently necessary to realize a high-performance, high-versatility solid state image sensor using a MOS type image sensor which can meet these demands.